1. Field of the Invention
The present invention relates to the control of exposure apparatus of the type equipped with a projection optical system and more particularly to an exposure light irradiation control system capable of ensuring a high degree of accuracy for the optical characteristics, e.g., magnification and focal position of such projection optical system.
2. Description of the Prior Art
With projection optical systems heretofore used with projection exposure apparatus adapted for the reduction projection of an image of a circuit pattern on a reticle to a semi-conductor wafer, a severe control is effected on the aberration, magnification, etc., of the optical systems during their manufacture. Particularly in the case of the reduction projection lens system, the exposure area on the wafer is on the order of 22 mm in diameter and the lenses used must be large in diameter so as to meet the required high resolution and large numerical aperture. As a result, the lens barrel itself is also increased in size considerably and the heat capacity is also increased.
In the case of ordinary lenses, no particularly serious problem is presented by any change of the optical characteristics such as the magnification due to the energy of light passed through the lenses.
However, in the manufacturing stage of ICs, particularly VLSIs, recently it has been required to reduce the linewidth of patterns and also a high degreee of accuracy has been required for the registration of patterns upon the wafer over the whole chip surface.
Therefore, if any error occurs in the magnification of the projected image of the next chip pattern to be printed with respect to the chip pattern printed on the wafer, even if an accurate registration is achieved for a part of the chip pattern, the registration accuracy of the other part is deteriorated extremely in accordance with the magnification error.
It is conceived that although differing in dependence on the optical lens arrangement of the projection lens system, such magnification error is caused by the temperature of the lenses themselves, the temperature of air between the lenses or the temperature changes within the lens barrel.